The molecular mechanisms of glomerular filtration are still largely unelucidated. This kidney function is affected in a large number of both acquired diseases such as diabetes nephropathy and nephrotic syndromes, as well as in hereditary diseases such as congenital nephrotic syndrome. These diseases, that can lead to end stage renal disease, are a major health problem worldwide. We have recently positionally cloned a gene mutated in congenital nephrotic syndrome. The gene product, named nephrin, is specifically expressed in the kidney where its expression appears to be confined to glomerular podocytes. They hypothesize that nephrin is generally involved in the pathomechanisms of acquired kidney diseases characterized by proteinuria and fusion of the epithelial foot processes. The specific goals of this project are to determine the biological role of nephrin and to explore its role in renal diseases. The Specific Aims are to: 1. Generate poly- and monoclonal antibodies against nephrin to study its cellular location and expression during development and in renal diseases. Furthermore, its molecular properties will be assessed using monoclonal antibodies. 2. Generate a mouse model for congenital nephrotic syndrome by inactivating the nephrin gene in embryonic stem cells by homologous recombination. 3. Study the intracellular signaling of nephrin, a putative membrane receptor. 4. Examine extracellular interactions of nephrin which is a putative transmembrane protein. 5. Study expression of nephrin in animal and human diseases of the kidney. The undertaking of the proposed work may add to our understanding of the glomerular filtration process, the pathomechanisms of proteinuria, as well as providing new diagnostic and even therapeutic means for this important kidney malfunction.